Minimal quasisections of circle bundles

Abstract

Let $E\stackrel{\pi }{\to }B$ be an oriented circle bundle over an oriented closed surface, and let $\mathcal{E}$ denote its Euler number. A \textit{quasisection} is a smooth surface $Q$, possibly with boundary, together with a generic smooth map $q:Q\to E$ such that $\pi \circ q(Q)=B$. The \textit{local formula} expresses the Euler number $\mathcal{E}$ of a bundle as a sum of the weights of the \textit{singular vertices} of a quasisection. In the present paper determine the minimal number $\mathcal{N}$ of singular vertices for closed and for bordered quasisections for a given circle bundle. For bordered quasisections, \[ \mathcal N_{\mathrm b}(E\to B) =\left\lceil\frac{3|\mathcal E|}{2}\right\rceil. \] For closed quasisections, If $\mathcal{E}$ is even, \[ \mathcal N_{\mathrm cl}(E\to B) =\frac{3|\mathcal E|}{2}. \] If $\mathcal{E}$ is odd, \[ \mathcal N_{\mathrm cl}(E\to B) =4|\mathcal E|. \] We also provide examples of quasisections that realize these minimal values.